Quantum Cascade Lasers (QCLs) are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum. Quantum cascade lasers are unipolar semiconductor lasers that utilize optical transitions between confined electronic sub-bands (e.g., conduction or valence bands) of semiconductor heterostructures. As a result, the emitted photon energy is determined by the thicknesses of the wells and barriers and can be tailored by bandgap engineering.
Specifically, a quantum cascade laser includes a periodic series of thin layers of varying material composition forming a superlattice in its optically active region. The superlattice introduces a varying electric potential across the length of the device, meaning that there is a varying probability of electrons occupying different positions over the length of the device. This is referred to as one-dimensional multiple quantum well confinement and leads to the splitting of the band of permitted energies into a number of discrete electronic subbands. By suitable design of the layer thicknesses, it is possible to engineer a population inversion between two subbands in the system under applied bias which is required in order to achieve laser emission. Since the position of the energy levels in the system is primarily determined by the layer thicknesses and not the material, it is possible to tune the emission wavelength of quantum cascade lasers over a wide range in the same material system.
Additionally, in a unipolar quantum cascade laser, once an electron has undergone an intersubband transition and emitted a photon in one period of the superlattice, it can tunnel into the next period of the structure where another photon can be emitted. This process of a single electron causing the emission of multiple photons as it traverses through the quantum cascade laser structure gives rise to the name cascade and makes a quantum efficiency of greater than unity possible which leads to higher output powers than conventional semiconductor laser diodes.
The terahertz frequency range, which may be loosely defined as the wavelengths between 30 and 300 μm, has historically been characterized by a relative lack of convenient radiation sources, detectors and transmission technology. It remains one of the least developed spectral regions, although a surge of activity in the past decade has advanced its potential for applications including, but not limited to, astrophysics and atmospheric science, biological and medical sciences, security screening and illicit material detection, non-destructive evaluation, communications technology, high resolution remote imaging, and ultrafast spectroscopy.
However, the development of terahertz systems has been slow principally related to the source technology. Currently, there does not exist room-temperature, high-power, widely-tunable terahertz sources that are compact, inexpensive and suitable for production in large quantities. The power generated by solid-state electronic devices rolls off with frequency owing to transit-time and resistance-capacitance effects. As a result, the available power generated above 1 terahertz is well below the milliwatt level. Compact electrically-pumped terahertz photonic devices are limited to p-doped Germanium lasers that require strong magnetic fields and cryogenic cooling for operation and terahertz quantum cascade lasers that achieve population inversion between two electron subbands spaced by THz photon energies. While terahertz quantum cascade lasers have achieved remarkable progress over the past decade, there still requires cryogenic cooling thereby greatly diminishing the usefulness of such lasers.
An alternative approach to THz quantum cascade laser source design is based on nonlinear terahertz Difference-Frequency Generation (DFG) inside of a dual-wavelength mid-infrared quantum cascade laser. Such devices are referred to as THz DFG-QCLs in the following. The active region in these devices is designed to provide mid-infrared emission at two different frequencies and to have giant optical nonlinearity, associated with intersubband transitions, for difference-frequency generation processes inside of the laser cavity. The design of these devices is described in M. A. Belkin, F. Capasso, A. Belyanin, D. L. Sivco, A. Y. Cho, D. C. Oakley, C. J. Vineis, and G. W. Turner, “Terahertz quantum-cascade-laser source based on intracavity difference-frequency generation,” Nature Photonics 1, 288-292 (May 2007) and M. A. Belkin, F. Capasso, F. Xie, A. Belyanin, M. Fischer, A. Wittmann, and J. Faist, “Room temperature terahertz quantum cascade laser source based on intracavity difference-frequency generation,” Appl. Phys. Lett. 92(20), 201101-1-201101-3 (May 2008) and is patented as M. A. Belkin, F. Capasso, and A. Belyanin, “Methods and apparatus for generating terahertz radiation,” U.S. Pat. No. 7,974,325, issued Jul. 5, 2011.
These THz DFG-QCLs have previously demonstrated THz emission at room temperature. Their waveguide was designed to provide confinement for both mid-IR and THz radiation and was tailored to achieve modal phase-matching for efficient nonlinear frequency generation. However, these devices only provide relatively small THz power output and are highly inefficient as approximately 99% of all THz radiation generated inside of their laser cavity is lost due to absorption in the laser active region.